Computer Science 1.2 Memory & Storage Test

What are the 4 types of primary memory

RAM, ROM, cache, registers

what does ram stand for

random access memory

what does ram do

holds instructions/data that’s currently in use

works with cpu so that instructions can be stored & processed

data can be read, written to, and deleted

very fast to read & write

is ram volatile or non volatile?

volatile: data is not retained when power is off

what does rom stand for

read only memory

is rom volatile or non volitile

non volatile: data is retained when computer is turned off

what does rom do

it’s a type of main memory directly accessible by the CPU

holds the first instructions the computer needs to start up (bios & bootstrap)

faster than secondary storage but slower than ram

smaller capacity than ram

BIOS

Basic Input Output System

used when computer starts up to check if hardware is working

virtual memory

used if a program needs more memory than is available

space is reserved in secondary storage to act as an extension of main memory

temporarily stores data that isn’t currently being used, freeing up space in ram

3 potential problems with virtual memory

disk thrashing: lots of pages being swapped in & out of virtual memory = high rate of hard disk access

memory capacity reached: too much use of memory

computer lag: computer runs slower because processor has a wait time to be swapped between rem & hard disk

cache

very small very fast memory, stores frequently used instructions and data to make the computer run smoother

prevents the cpu from having to repeatedly fetch instructions from ram

why do we need secondary storage

ram is volatile (data erased when computer turned off) so we need a place to store data permanently

what is the main form of secondary storage on a computer

the hard drive

why do we need primary storage

access times are faster = fetch-execute cycle times are reduced

holds data & instructions needed by cpu

used as short term working memory for hardware directly connected to the cpu (e.g. ram) and components in cpu (e.g. cache & registers)

is secondary storage volatile or non-volatile

non-volatile: data is kept when computer is turned off

what is the main purpose of secondary storage

permanent data store for programs & files

can be built in or external

three different types of secondary storage

magnetic, optical, solid state

magnetic storage (secondary)

uses platters (discs) covered in special material to store data

sections of the material can be magnetised / demagnetised to represent the 2 binary states

a read/write head attached to an arm applied a magnetic field to specific sections to read (measuring whether sections are magnetised or not) and write (magnetising/demagnetising sections of the platter)

advantages of magnetic storage

• affordable for large capacities

• infinitely rewritable

• reliable for long periods of time

• faster read speeds than optical

disadvantages of magnetic storage

• slower than solid state

• requires lots of power

• not durable because of moving parts

• not portable

• can be noisy

optical storage (secondary)

small & affordable

uses reflective discs & light

laser burns marks into reflective surface of discs called pits, and they reflect light representing binary value 1. the spaces in between the pits are lands, which don’t reflect light & represent binary 0

examples include CDs, DVDs & Blu-Ray

how optical storage is read/written

read: laser is set to lower power & shone on disk, light sensor reads whether light is reflected or not

write: disks spin & laser burns pits into reflective surface

three formats of optical storage

read only (ROM)

write once (W)

rewritable (R)

advantages of optical storage

• affordable

• small & portable

• lasts a long time (if stored right)

disadvantages of optical storage

• expensive for larger capacity

• easily scratched

• slow read speeds

• limited reusability

solid state storage (secondary)

record data using transistors that retain their state even with no power

if a transistor contains elections it equals 0 and if it doesn’t it represents 1

to write, an electrical charge is used to create a binary string which causes the electrons to move between the two states

there are no moving parts, only electronic ones

advantages of solid state storage

• durable (no moving parts)

• fast read/write speeds

• no noise

• portable

• high capacity (but not highest)

disadvantages of solid state storage

• expensive

• less storage capacity than magnetic

factors to consider when choosing a storage device

• capacity

• speed

• cost

• portability

• durability

• reliability

bit

• smallest unit

• symbol b

• either a 1 or 0

nibble

4 bits

byte

8 bits

symbol B

a single character (a, 2, g)

kilobyte

• a thousand bytes

• symbol KB

• e.g. a small text file

megabyte

• MB

• a million bytes

• e.g. a music file

gigabyte

• GB

• a billion bytes

• e.g. a HD movie

terabyte

• TB

• a trillion bytes

• e.g. a large hard drive

petabyte

• PB

• a quadrillion bytes

• a large data centre

why does everything need to be converted to binary for a computer

computers operate in binary systems - something is either on or off - so information needs to be in binary for the computer to understand it

data capacity

the max amount of information that a storage device can hold

formula for file size of text file

bytes per character x number of characters

formula for size of image files

resolution x colour depth

(resolution = width x height)

formula for size of sound files

sample rate x duration in seconds x bit depth

denary

our number system - 10 digits

it’s a base 10 number system, as each digit has a weight factor of 10 raised to a power

binary

a number system made up of 2 digits: 0 and 1

a base 2 number system: each digit has a weight of 2 raised to a power (starting with 2⁰, 2¹, 2² etc)

to represent bigger numbers we add more binary digits (bits)

computers use binary to represent dara with transistors which can be only either on or off

how to convert denary → binary

1. write out binary headings 1, 2, 4, 8 etc (more if you need, double each time) up to 128 normally

2. start with 128 and see if f your number goes into each heading or not, write a 0 or 1

3. move along the rows to the right, subtracting the column heading from the denary value until you have a number under each heading

binary → denary conversion

1. write out headings 1, 2, 4, 8 etc (more if you need, double each time)

2. write the binary number with one digit under each heading

3. add together any value with 1 under it

binary golden rules table

what is overflow error with binary addition

when the result of a binary addition exceeds the available bits, and would need another bit to represent it

hexadecimal

number system made up of 16 digits: 10 numbers (0-9) and 6 letters (A-F) representing numbers 0-15

base 16 number system: each digit has weight of 16 raised to a power

one hex digit can represent four bits of binary data (a nibble)

why is hexadecimal used

used when working with large values as it takes fewer digits to represent values

binary is harder to read because there are more bits

binary can also be prone to errors when being copied

converting denary → hex

1. convert it to binary

2. split binary number in half (into 2 nibbles)

3. figure out what each nibble is in hex and put them together to make the hex number

how to convert hex → denary

1. convert each of the 2 hex digits into binary nibbles

2. put them together into one long binary number

3. change the headings into the normal binary ones and convert the binary number to denary

binary shifts

how computers perform basic multiplication & division

binary digits are moved left or right a set number of times

a left shift multiplies a binary number by 2

a right shift divides a binary number by 2

a shift of 2 places would multiply/divide by 4, and so on

any empty columns are filled with a 0

problem with binary shifts

can cause a loss of precision by discarding bits, leading to changes in the numerical value

sometimes overflow & underflow errors can occur too

most significant bit

the bit in a binary number that has the highest value (the leftmost digit)

least significant bit

the bit in a binary number that has the lowest value (rightmost digit)

character set

a list of characters recognised by the computer, with each character being represented by a single number. logically ordered

ASCII

standard code used worldwide for characters

7 bit binary codes

includes punctuation

8th bit added later on for other characters e.g. emojis, symbols, different languages - this is called extended ASCII

logically ordered

normal ASCII only represents basic characters in english - limited use

Unicode

character set used for other alphabets for different languages - solution for ASCII

16 bits

has every character in every major language plus other characters e.g. emojis

but uses more storage space than ASCII

bitmap images

images made out of pixels

each pixel is a single colour, given a binary value to represent it

pixel definition

the smallest identifiable area of an image (picture element)

image resolution

the concentration of pixels within a specific area, defined by the image width & height in pixels

metadata (for image)

metadata is data about data

for images this is the width, height, and bits per pixel

bits per pixel

the number of digits in each of the values assigned to each pixel in a image

the more bits per pixel, the more colour combinations available

colour codes

all colour is made of different values or red, green, and blue

each has a 24-bit colour depth

expressed using HEX values

effect of colour depth & resolution on file size

higher colour depth / resolution = larger file size

sound definition

a pressure wave that causes the air to vibrate. our eardrums then vibrate, allowing the brain to interpret this as sound

analogue sound signals

continuous sound signals, natural sound, computers can’t interpret them

digital sound signals

discrete sound signals, so can be converted into numbers that represent aptitude. can be interpreted by a computer as it can be digitalised (turned into binary digits)

how speakers work

speakers allow devices to generate sound from binary numbers / digital signals

a vibrating cone in the speaker causes pressure waves in the air, which we hear as sound

amplitude of a wave & what it shows

the height of the wave, representing the volume

the higher it is, the higher the volume of the sound

frequency of a wave & what it shows

how close together the waves are, shows the pitch

the closer together the waves, the higher the pitch (note)

how sound is stored

sound recorded using a device e.g. a microphone that can turn sound waves into an electrical signal

measurements of the level of signal (samples) are taken at regular intervals. they are then converted into binary

sample resolution / bit depth of sound

the number of bits per sample

by increasing sample resolution, you can sample even more of the original analogue wave, increasing quality of the representation

two ways to increase the quality of sound

1. increase sample rate

2. increase sample resolution

but both of these will make the file size larger

what is sample rate measured in

hertz Hz

bit depth of a sound file

number of bits available to store each sample, the more there are the better quality the sound is

compression

reducing the size of a file, meaning it takes up less space, making data storage on devices more efficient

how is compression useful

quicker transfers over the internet because of smaller file sizes

increased download speeds

voice can be transmitted fast enough to keep up with speech

two types of compression

lossy: non essential data is permanently removed

lossless: patterns in data are spotted & summarised, no permanently removed data

lossy compression

• good for image files & audio files

• makes similar coloured pixels all the same colour, removing unnecessary tones

• range of colours contained by the image is reduced

• for audio bit depth is reduced

• but uncompressed data won’t be the same as original file

• not suitable for text!

• better at reducing file size than lossless

lossless compression

• no data lost

• finds groups of repeating data, recording the data only once, with the number of times it was repeated

• ideal for text as it finds patterns & encodes it in a dictionary so it can be used again

• not as good at reducing file size than lossy

• uncompressed file is the same as the original

which type of compression for which type of file

lossless: text files & executable code

lossy: audio & images